In known steam irons of this type, the throttle is of generally cylindrical shape and is mounted gripped and passing through the passage of the outlet and its bulge opens permanently into the steam chamber; the throttle comprises a longitudinal groove over a portion of its length and is movable axially to occupy either the dry position in which the two ends of the groove are located on the same side of the passage, thereby preventing any passage of water, or its steaming position in which the two ends of the groove are located on opposite sides of the passage, thereby permitting the passage of the water into the steam chamber via the groove in the throttle. However, because the throttle is in surface contact over all the height of the passage of the outlet, each manipulation of the throttle by the user creates high stress in the outlet which operates at a relatively high temperature. Because of this stress to which the outlet is subjected, the pressure drops about the throttle are relatively high, such that when the throttle is in its steaming position, also called the steam position, the flow rate of water passing continuously into the vaporization chamber is particularly irregular, and this the more so as the level of water in the reservoir falls in the course of ironing; the continuous flow rate of steam which results is thus itself very irregular, thereby decreasing the quality of the pressing.